Spinal cord injury (SCI) causes severe bone loss which may result in pathologic fractures and, consequently, increased morbidity and health care costs. To date, the most effective strategies for preserving or restoring bone after SCI have utilized electrical stimulation (ES)-induced musculoskeletal loading. One critical question is whether the beneficial effects of ES reflect a decrease in the elevated rate of bone resorption, increased formation of new bone, or both. Answers to these questions are likely to guide the development of future strategies to minimize bone loss after SCI by identifying those components of bone metabolism that are not optimally stimulated by ES and might therefore be targets for additional types of intervention. Our preliminary data indicate that one week of reloading of bone after SCI by ES reduced bone resorptive activity through the inhibition of osteoclastogenesis and osteoclast activity in a rat model of SCI. Importantly, ES reversed SCI-induced upregulation of the expression in osteoblasts of the Wnt inhibitors (DKK1, sFRP2 and SOST) and increased expression in these cells of the Wnt-responsive gene osteoprotegerin (OPG), an inhibitor of the differentiation and activity of osteoclasts. SCI related-bone loss may be worsened by reduced circulating levels of testosterone because this steroid hormone has anabolic actions on bone and its levels are commonly reduced after SCI. Recently, we observed that nandrolone, an anabolic steroid, reduced bone loss after paralysis due to SCI. Interestingly, nandrolone increased the expression of OPG, Runx2 and LRP5 in bone marrow-derived osteoblasts from SCI rats. LRP5 is a Wnt co-receptor closely linked to the control of bone mass, and Runx2 is another Wnt-responsive gene, and is involved in osteoblast differentiation. These findings suggest that one effect of nandrolone in unloaded bone is to stimulate Wnt signaling in osteoblasts and that Wnt signaling explains at least some of the protective effects of nandrolone on bone after SCI. The central hypotheses suggested by our findings are: 1) ES effects on bone are due to a decrease in the elevated rate of bone resorption and increased formation of new bone; 2) anabolic steroids will provide additional or synergistic benefits to SCI-related bone loss when applied in conjunction with ES. If this is the case, these findings would be readily translated to clinical cae, and would provide an innovative therapeutic strategy for the marked loss of bone following SCI. As such, the following specific aims are proposed: Specific Aim 1. To determine how ES alters the rates of bone formation and resorption and the properties of bone cells, and to characterize the effects of ES on bone mass. We will determine how longer or higher intensity of ES (i.e., 3 months for one hour a day or 1 month for 3 hours a day) alters BMD and BMC, and both metabolic and histomorphometric measures of bone resorption and bone formation. We will also test how longer periods or higher intensity of ES alter numbers of osteoclasts and osteoblasts in cultures of bone marrow cells and examine changes in expression in these cells of genes for their differentiation and activity. Specific Aim 2. To determine whether a combination of ES and an androgen will reduce SCI- related bone loss to a greater extent than either ES or androgen alone. Nandrolone will be administered with or without concurrent ES. We will compare the effects on bone and bone cells of nandrolone combined with ES with the effects observed for ES or nandrolone alone. The possible role for other cellular and molecular mechanisms by which SCI-related bone loss is reduced by ES alone or nandrolone alone or in combination will be studied using DNA microarry analysis.